The present invention relates to a trigger circuit for a thyristor and in particular to a circuit that will trigger a thyristor into conduction when the voltage appearing across the thyristor has a predetermined polarity and exceeds a predetermined magnitude.
It is common to use thyristors and in particular silicon controlled rectifiers (SCR's) to selectively connect a load to an a-c power source. The SCR can be switched from the non-conductive state to the conductive state by applying a trigger signal to the gate input when the SCR anode is more positive than the SCR cathode. In a prior art gate control circuit the SCR gate trigger signal is derived directly from the a-c power signal at the SCR anode. The gate trigger circuit is responsive to the voltage at the SCR anode and generates sufficient gate current to trigger the SCR into the conductive state once each cycle when the SCR anode voltage is positive with respect to the SCR cathode and exceeds a predetermined magnitude. Examples of such gate control circuits are illustrated and described at pages 91-94 of the SCR Manual Fifth Edition (1972), published by the General Electric Company.
In another prior art SCR gate trigger circuit as described in U.S. Pat. No. 3,793,537, Stringer, a separate trigger signal is combined with a signal representative of the polarity of the voltage across the SCR to generate a SCR gate trigger signal if the separate trigger signal occurs when the SCR anode voltage is more positive than the SCR cathode voltage. Stringer uses a differential amplifier for sensing the polarity of the voltage across the SCR and uses the differential amplifier output to enable the circuit which generates the SCR gate trigger signal.
Applicant has found that in certain applications it is desirable to enable the gate trigger circuit of the SCR based on both the polarity and the magnitude of the voltage appearing across the SCR. The reason for this is that on a short time basis SCR's may be generally considered to be charge control devices (see page 88 of the SCR Manual, supra). When a SCR gate trigger pulse is present long enough for the free charge in the SCR base region to reach a certain level and the SCR anode is positive with respect to the cathode, the SCR will be triggered into conduction. But if the SCR anode is not positive with respect to the SCR cathode during the occurrence of the SCR gate trigger pulse, it will take a short period of time for the free charge level in the SCR base region to be reduced to a level that will reliably maintain the SCR in the non-conductive state. The application of a forward voltage across the SCR before the free charge level in the SCR base region is significantly reduced is similar to firing the SCR with a weak gate signal which could result in triggering the SCR into conduction at an unwanted time, or may result in possible damage to the SCR. This condition could arise if the gate trigger signal for the SCR is not directly derived from the a-c power signal being switched by the SCR, or if the gate trigger signal is not synchronized or loses synchronization with the voltage appearing across the SCR.
It is, therefore, an object of this invention to provide a simple circuit for triggering a SCR into conduction when the forward voltage across the SCR has a desired polarity and exceeds a predetermined magnitude.
It is another object of this invention to switch a SCR to the conductive state upon the occurrence of a trigger signal when the forward voltage across the SCR is positive and exceeds a predetermined magnitude.